1. Field of the Invention
Exemplary embodiments of the invention relate to light emitting diodes and, more particularly, to light emitting diodes with improved luminous efficiency.
2. Discussion of the Background
Gallium nitride (GaN) based light emitting diodes (LEDs) have been used in a wide range of applications including natural color LED display devices, LED traffic sign boards, white LEDs, etc.
The GaN-based light emitting diode is generally formed by growing epitaxial layers on a substrate, for example, a sapphire substrate, and includes an n-type semiconductor layer, a p-type semiconductor layer, and an active layer interposed between the n-type semiconductor layer and the p-type semiconductor layer. Further, an N electrode pad is formed on the n-type semiconductor layer and a P electrode pad is formed on the p-type semiconductor layer. The light emitting diode is electrically connected to and operated by an external power source through these electrode pads. Here, electric current may be directed from the P-electrode pad to the N-electrode pad through the semiconductor layers.
Generally, since the p-type semiconductor layer may have high specific resistance, electric current may not be evenly distributed in the p-type semiconductor layer, but may be concentrated on a portion of the p-type semiconductor layer having the P-electrode formed thereon, and may cause a problem of current crowding at an edge of the p-type semiconductor layer. Current crowding may lead to a reduction in light emitting area, thereby deteriorating luminous efficiency. To solve such problems, a transparent electrode layer having low specific resistance may be formed on the p-type semiconductor layer so as to enhance current spreading. However, despite excellent electrical conductivity of the transparent electrode layer, current crowding may occur near the electrode pad, and although the transparent electrode layer may induce some degree of reflection or refraction of light, optical loss may occur due to total internal reflection caused by a difference in refractive index between the transparent electrode layer and the exterior.
In this general structure, electric current supplied from the P-electrode pad may be dispersed by the transparent electrode layer before entering the p-type semiconductor layer, thereby increasing a light emitting area of the LED. However, since the transparent electrode layer may absorb light, the thickness of the transparent electrode layer is limited, thereby providing limited current spreading. In particular, in a large-scale LED having an area of about 1 mm2 or more, there may be a limit in achieving efficient current spreading through the transparent electrode layer.
To assist current spreading, a large-scale LED may include pad extensions extending from the electrode pads. For example, U.S. Patent Application Publication No. 2010/0044744, applied for by Kim, et al., discloses an LED which includes pad extensions to enhance current spreading. Conventionally, the LED is provided at an upper side thereof with a transparent electrode layer in addition to the pad extensions for current spreading. The transparent electrode layer assists current spreading in cooperation with the pad extensions.
However, pad extensions and a plurality of n-electrode pads occupying a relatively large area are generally formed by etching an active layer and an upper semiconductor layer which have a relatively wide area. Accordingly, the formation of the n-electrode pad and the pad extensions may result in a reduction in light emitting area, causing deterioration in light emitting efficiency. Alternatively, although the LED may include a plurality of n- and p-electrode pads to assist current spreading, the formation of the plural electrode pads may lead to an increase in the number of processes such as a wire-bonding process, thereby decreasing package yield. Furthermore, since the n-electrode pad and the p-electrode pad are disposed to face each other, current crowding is likely to occur between adjacent electrode pads, thereby causing uneven light emission from a central region of the light emitting diode.
On the other hand, a patterned sapphire substrate may be used to improve LED light extraction efficiency. The pattern on the sapphire substrate scatters or reflects light generated in the active layer to reduce optical loss due to total internal reflection inside the LED, thereby improving light extraction efficiency.
Although improvement in light extraction efficiency may be achieved by use of the patterned sapphire substrate, the relatively high refractive index of the GaN-based compound semiconductor may result in optical loss by total internal reflection inside the LED.